Acoustic Stethoscope



H. F. OLSCN ACOUSTIC sTETHosCoPE Nov. 28, 1944.

NOV. 28, 1944. .H, F OLSON 2,363,686

ACOUSTIC STETHOS COPE FEOl/EMY//V CYCLES WA SHO/l0 lhwentor attorney Hf roban,

toRadioCorporaticn Haddon Heights, N. J.. alsignor ci' America, a corporation f j' Application apen 1. 1942, semi No. 43.7.139

` (ci. isi- 24) Claims.

This invention 'relates to acoustic pickup devices and more particularly to an acoustic stethoscope. i

The physieians stethoscope is one of the most useful instruments which he has in diagnosing a patients condition. To be most useful, this instrument should reproduce accurately and eiliciently the body sounds which it picks up. There are' two types of stethoscopes in'use generally, one being the open bell type, and the yother the diaphragm type. The latter is usually preferred primarily because it .excludesor attenuates external noises, andl because it avoids leakage between the body and the stethoscope.

Investigation has shown that conventional stethoscopes are rather imperfect devices when judged from the standpoints of accuracy and efflciency. The range of sounds which are transmitted by the existing open bell type stethoscopes is about 30 to 1200 cycles. The existing diaphragm type stethoscopes cover a range of from only about 250 to 1200 cycles. lOn the other hand, the sounds of the human body range from about 40 to 4000 cycles. Another disadvantage of prior art stethoscopes is that practically all of them have a response which is fairly ragged, these stethoscopes introducing distortion in the form of frequency discrimination in the steadystate sounds and relative accentuation of certain frequencies in the case of transient sounds.

The primary object of my present invention is to provide an improved stethoscope which is not subject to the aforementioned defects and limitations characteristic of prior art stethoscopes.

More particularly, it is an object of my present invention to provide an improved acoustic stethoscope by means of which the sounds within the human body or other subjects under study may be distinctly heard.

Another object of my present invention is to provide an improved acoustic stethoscope as aforesaid which has a much wider frequency range than similar stethoscopes heretofore used.

Still another object of my present invention is to provide an improved stethoscope as aforesaid which has a smooth response over its entire working range.

A further object of my present invention is to provide an improved acoustic stethoscope which will not pick up sounds o'r noises extraneous to the subject under study.

Still a further object of my present invention is to provide an improved acoustic stethoscope which is freefrom transition losses.

f It is also an object of my present invention to provide an improved acoustic stethoscope as above set forth which is simple inconstruction, inexpensive in cost, and highly eflicient in use.

It is well known in the acoustic art that-maximum transfer of power from one medium to another occurs when the ,impedances are matched. To accomplish this is usually not as easy as it would seem. It is for this reason that the acoustic stethoscope has retained, for all intents and purposes, the same essential design down through the ages. For example, the acoustic impedance of each ear canal is approximately the same as the surge acoustic impedance of a tube filled with air having an area of about `0.2 square centi meter. For both ears, the acoustic impedance of both ear canals is approximately the same as the surge acoustic impedance of a tube having an area of 0.3 square centimeter, or a diameter of about V1 inch. To match the air in a V4 inch diameter tube to the acoustic impedance of the body, which is about 300 times that of the air of the tube in this range, the pickup diaphragm must be 300 times that of a tube V4 inch in diameter, or about 41A inches. This is entirely too large for a stethoscope because it would be impossible to localize sounds with a stethoscope having a pickup diaphragm of 4V4 inches in diameter. A compromise must be made, therefore, and for this reason stethoscopes have been provided with diaphragms of about .1V2 to 2 inches in diameter, with a resultant mismatch of about 8 to 1 for the 1V2 inch diameter and about i1/2 to 1 for the 2 inch diameter. This is bad mismatch and represents a sizable loss. Moreover, even a 1V2 to 2 inch diameter diaphragm is still too large. About the maximum tolerable diameter for the pickup end of a stethoscope is 1 inch. To match this diaphragm in contact with the body to air in a tube, the diameter of the tube would be slightly less than inch. However, this does not match the ear impedance because the surge acoustic impedance of a tube inch in diameter is about ten times the impedance of the ear canal or twenty times the impedance of v both ear canals. According to one of the features of my invention, I use a tapered tube to match the relativelyhigh impedance atthe pickup end of a stethoscope with a 1 inch diaphragm to the relatively low impedance of the ear. By this expedient, I can employ a small diaphragm which is most useful in localizing the source of sounds and at the same time match the relatively high impedance of the body to the relatively low impedance of the ear canal.

To merely couple one diaphragm to a system as outlined above is not enough. I find that stethoscopes heretofore used are deficient in low frequency response over and above the mismatch in acoustic impedance. This is due to the relatively stiif diaphragms employed in prior art stethoscopes. At low frequencies, the acoustic impedance ofthe conventional, stiff diaphragm is much greater than the body. The result is at tenuation of the low frequencies. The logical solution would be seen to be a limp diaphragm.

However, in order to employ a limp, simple diaphragm, the spacing back of the diaphragm must be large in order to obviate closure of the tube by the diaphragm when the stethoscope is pressed to the body. The spacing introduces a shunt acoustic capacitance which, when large, shunts out the high frequencies. According to another feature of my invention, therefore, and in order to retain very small clearance and at the `same time obtain a diaphragm with small inherent stiiness, I support the diaphragm on a multitude of resilient projections. This type of diaphragm and support will transmit low frequencies without attenuation because the stiffness is relatively small. At the same time, this type of diaphragm will transmit high frequencies without attenuation because the spacing between the diaphragm and back plate is small, which, in turn, means that the shunt lacoustic impedance is large because the shunt acoustic capacitance is small. The stethoscope designed in accordance with my present invention exhibits a uniform response over a frequency range of from about to about 4000 cycles. Furthermore, the sensitivity of my improved stethoscope is about 15 decibels greater than that of the conventional open bell type stethoscope and about 6 decibels greater than that of existing diaphragm type stethoscopes.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will be best understood from the following description of one embodiment thereof, when read in connection with the accompanying drawings, in which,

Figure 1 is a chart showing the range of various sounds generated in the human body, the frequency range of the fundamentals being shown in solid bars and the frequency range of the harmonies being indicated by the sectioned bars;

Figure 2 is an elevational view, partly in section, of an acoustic stethoscope constructed in accordance with this invention;

Figure 3 is a schematic cross-sectional view of the acoustic system of my improved stethoscope shown applied to the body and the ear;

Figure 4 is a wiring diagram of the equivalent electrical circuit of the acoustic system shown in Fig. 3;

Figure 5 shows a set of curves representing the resistive and reactive components of the acoustic impedance looking into the ear canal;

Figure 6 is a set of response curves showing, by way of comparison, the responses of (I) the conventional open bell type stethoscope, (II) the conventional diaphragm type stethoscope, and (III) my improved stethoscope; and

Figure 7 is a plan view, partly broken away, of the backing member constituting a part of the pickup device of my improved stethoscope.

Referring more particularly to the drawings, wherein similar reference characters indicate coraseaose frequency band of some of the most common sounds of the human body. The fundamentals are shown as dark areas and the harmonics or overtones as cross hatched areas. The fundamental of the systolic sounds range from to 80 cycles. The overtones are scattered over the remainder of the frequency band up to 4000 cycles and over. Above 4000'cyc1es, most of the sounds in the body are so weak that they are masked by the general random noises generated in the body. The fundamental diastolic sounds range from 60 to 100 cycles. The overtones are scattered over the remainder of the frequency band up to 4000 cycles. The fundamental sounds of systolic and diastolic murmurs range from 300 to 800 cycles. The overtones in certain cases can be observed up to 2000 or 3000 cycles. Prestolic murmurs are usually of a low frequency, ranging from about 60 to about 200 cycles. The overtones range up to about 1000 cycles. Above this frequency, the overtones are masked by the body sounds. The fundamental of peristaltic sounds has a tremendous range in both frequency and intensity. Fundamentals up to 2000 cycles are quite common. The overtones in the case of very intense sounds extend beyond 4000 cycles.` The fundamental frequency of respiratory squeaks, rales, crackles, and groans ranges from 40 cycles to 1000 cycles. Respiratory sounds such as wheezes and the rushing of air are of a random nature and do not possess true fundamentals. The components of these sounds are scattered over the entire audible spectrum.

Since diagnosis is based on the structure of the sounds, it is very important that the stethoscope should transmit all frequencies over the range from about 50 to 4000 cycles without attenuation or discrimination in order to obtain maximum results, as is evident from Fig. 1. Such results can be obtained with the stethoscope of Fig. 2 which shows a pickup device I adapted to be placed against the human body or other subject to be studied. The pickup device I comprises a supporting plate 2 having a hollow stem 3 exresponding parts throughout, Fig. 1 shows the 75 tending from its back face, a bore or opening 5 being formed in .the supporting plate 2 in communication with the hollow stem 3. Secured to the front face of the plate 2 is a backing member l having an opening 3 in communication with the opening 5 and having a plurality of forwardly extending projections II thereon. The backing member I may be of any suitable material, but I-prefer to make it of an elastic material, such as rubber. The proj tions Il may be of any suitable shape, such as conical, pyramidal, or the like, being shown as small pyramids with curved sides or faces as one example of the type of projections found satisfactory. A mem rancus diaphragm I3 of thin, sheet rubber o the like is carried by the supporting plate 2 wi its rear or inner surface in engagement with `e apices of the projections II. The projections II `are spaced from each other on the backing member 1 and are distributed over the entire area of the diaphragm I3, the spacing of the projections being such as to provide a plurality of intersecting and intercommunicating passageways I4 which communicate with the openings l and 5 and the hollow stem 3.

Fitted onto the stem 3 is a flexible tube I5 of rubber or the like which connects the pickup device I to a Y connector I1, the latter connecting the tube I5 to a pair of ear tubes I9 terminating in a pair of ear pieces 2|. The tube I5 is provided with a tapered passage 23 the stem. fto the connector I'I. The connector I'I and the ear .tubesj'are formed withY similarly expandinges 23 and-21 so that, from thesfem 3y to pieces I I, a continuously expanding passage is provided. The Adlapln-agm II is ot the order-.of one inch in diameter whereby its-areaisofthe order of scuare inchA and thefidiameter of the en o 'o at the stem I -is 'approximately inch whereby its cross sectionalarea-at stemt is of the order of f., 1024 K square inch. Thus, the impedance of'the area of the human body covered by the diaphragm I3,when in lcontact therewith matches the surge acoustic impedance of the air in the passageway 23 at its smaller-end. Since, however, this does not match the impedance o1' the ear canals,

as' represented by the curves ne and .me of are very short, being ofthe order of $410 inch.

Thus, the clearance or space behind the diaphragm I3 represented by the intersecting passageways I4 is very small. In this Way, the diaphragm has small inherent stiffness and therefore will transmit low frequencies without attenuation, while at the same time transmitting high frequencies without attenuation because the space between the diaphragm and the backing member 'I is small.

Fig. 3 shows a simplified acoustic system of the body B which is under study or examination and has a sound source S, a stethoscope according to my present invention, and the ear E. Fig. 4 shows a wiring diagram of the analogous electrical system or circuit. In the latter figure, ZAB represents the impedance of the body B, ZAE represents the impedance of the ear canals, M is the mass of the diaphragm I3, CAD is the capacitance of the diaphragm, Cile isthe capacitance of the resilient projections II, and CAA represents the capacitance of the air chamber between the diaphragm I3 and the resilient projections or supports II, p being the acoustic pressure applied to the diaphragm I3. It will be noted that the tapered passage or line 23, 25, 21 acts as an acoustic transformer, being expandingly tapered all the Way from the pickup device I to the ear piece 2I and the ear E. This tapered line is represented by the block 30 in Fig. `4 representing a transformer.

The measured frequency response characteristics of conventional stethoscopes and of my improved stethoscope are shown by the curves of Fig. 6, the dash line curve I representing the response of the conventional bell type stethoscope, the dot and dash line curve II representing the response of the conventional diaphragm type' stethoscope, and the solid line curve III representing the response of my improved stethoscope. It will be noted that the output of my asoaese 3 winch expands muuuy andunirormiy from improved stethOwODe is about I2 decibels higher than the conventional bell type stethoscope and about 8 decibek higher than the conventional diaphragm type stethoscope in its most sensitive region. It will also be noted that my novel stethoscope has considerably greater high frequency and low frequencyv ranges than do existing stethoscopes. Whereas existing stethoscopes cut ofi' at around 1500 cycles, the high frequency response of my improved stethoscope is maintained to about 4000 cycles. It will further be noted that the response of my improved stethoscope is quite uniform, whereas the curves I and -II show that the response of prior art stethoscopes is non-uniform. The peaks" and dips" in the response frequency characteristic of each of the bell type and diaphragm type stethoscopes of the prior art indicate frequency discrimina.- tion. Thus, for example, if the fundamental of a murmur occurs at 300 cycles, either type of existing stethoscope will accentuate this as comparedvto another murmur with'a fundamental at 400cycles. If both murmurs were at exactly the same intensity, the prior art stethoscopes would show a difference of 8 decibels, or an intensity difference of.6 1. A system with a ragged response characteristic such as represented by the curves I and l1 will exhibit very poor transient response, that is, response to impulsive sound. Instead of conveying a true sound, it will transmit resonant `frequencies of the system. It is quite evident, therefore, that uniform response of my improved stethoscope would help greatly in diagnosis because the entire analysis is based on the character of the sound.

From the foregoing description, it will be apparent to those skilled in the art that I have provided a lnovel stethoscope which will faithfully transmit sounds originating in the human body or other subject under study. Although I have shown and described but one embodiment of my invention, it will be apparent to those skilled in the art that many other modifications thereof, as Well as changes in the one described,

are possible. I therefore desire that my invention shall not be limited except insofar as is made necessary bythe prior art and by the spirit of the appended claims.

I claim as my invention:

1. In an acoustic stethoscope, a pick-up device having an impedance substantially equal to that of a given area of a subject to be examined, and an acoustic coupling line connected at one end to said pick-up device and adapted to be connected at its other end to a detector having a different impedance than said first named lmpedance, said line having a passage of gradually varying cross section and the ends of said passage having impedances which match said first and second named impedances, respectively, whereby said coupling line transmits acoustical energy without substantial loss.

2. In ani acoustic stethoscope, a pick-up device having a relatively high impedance equal substantially to that of a given area of a subject to be examined, and an acoustic coupling line connected at one end to said pick-up device and adapted to be connected at its other end to a detector having a relatively low impedance, said line having a tapered passage between its ends which expands gradually from said pick-up device to its detector end, and the ends of said passage having impedances which match said high and low impedances, respectively, whereby said coupling line transmits acoustical energy without substantid attenuation.

3. The invention set forth in claim 2 characterized in that said passage varies uniformly in cross section from one end to the other.

4. In an acoustic stethoscope, a pick-up device including a relatively limp, exible vibratory member adapted to engage a given area of a subjectto be examined, said subject area having a relatively high impedance, and an acoustic coupling line connected to said pick-up device and adapted to be connected at its other-end to a detector having a relatively low impedance, said line having a tapered passage between its ends which expands gradually from said pickup device to its detector end, and the ends of said passage having irnpedances which match saidhigh and low impedances, respectively, whereby said coupling line transmits acoustical energy without substantial attenuation.

5. In an acoustic stethoscope, a pick-up device including a relatively limp, flexible vibratory member adapted to engage a given area of a subject to be examined and a backing member having a plurality of projections in engagement with said vibratory member, said subject area having a relatively high impedance, and an acoustic coupling line connected to said pick-up device and adapted -to be connected at its other end to a. detector having a relatively low impedance, said line having atapered passage between its ends which expands gradually from said pickup to its detector end, and the ends of said passage having impedances which match said high and low impedances, respectively, whereby said coupling line transmits acoustical energy without substantial attenuation.

d. The invention set forth in claim 5 charactery asoaeso 11. An acoustical piek-up device comprising a supporting member,v a backing member o! yieldable material carried by said supporting member and having a plurality of projections thereon, andra diaphragm on said supporting member having one surface thereof vin engagement with said projections, said projections being distributed over the area of said surface and being spaced from each other to provide a. plurality of intersecting, intercommunicating passageways therebetween.

12. An acoustical pick-up device according to claim 11 characterized in that said backing member is made of an elastic material.

13. An acoustical pick-up device according t0 claim 11 characterized in that said backing member is made of rubber.

ized in that said vibratory member is constituted 4o each other to provide intercommunicating acoustic passageways therebetween and are distributed over the area of said member, and characterized further in that said intercommunicating passageways have communication with said tapered passage.

10. An acoustic stethoscope comprising a pickup element matched in impedance to the subject to be explored thereby, an ear piece element matched in impedance to the ear canals, and means providing a tapered passageway constituting an acoustic transmission path coupling said elements, said passageway being of smallest cross sectional area at the pick-up element end thereof and of largest cross sectional area at the ear piece element end thereof, and said passageway having an impedance at each end, looking into the respectively associated elements, of substantially the same magnitude as said respective elements throughout the range of sounds to be transmitted thereby.

14. An acoustical pick-up device according to claim 11 characterized in that said projections are substantially pyramidal in shape, and characterized further in that their apices are in engagement with said diaphragm.

15. An acoustical pick-up device according to claim 11 characterized in that each of said projections is tapered to substantially a point, and characterized further in that the pointed ends of said projections are in engagement with said diaphragm.l 16. An acoustical pick-up device according to claim 11 wherein said projections have a length of the order of 1,550 inch.

1'?. An acoustical pick-up device according to -claim l1 characterized in thatsaid diaphragm is constituted by a thin, rubber membrane in substantially unstretched condition.

18. An acoustical pick-up device according to claim 11 characterized in that said diaphragm has an area of the order of 0 way coupled at one end to said diaphragm and adapted to be connected at its other end to a detector having a different impedance from that of said source, said means constituting an acoustic transformer which matches the impedance of s said source to the impedance of said detector whereby said transformer is capable of transmitting acoustical energy from said source to said detector without substantial attenuation.

20. Acoustic apparatus according to claim 19 0 wherein said diaphragm has an area of the order of square inch, and wherein said passageway has an area of the order of cent to said diaphragm.

HARRY F. OLSON. 

